In general, a construction machine including an excavator comprise work apparatus such as a boom, an arm, and a bucket, which is installed on an upper swing structure mounted on a lower traveling structure. The movement of the work apparatus and the upper swing structure is controlled so as to be operated in cooperation with each other so as to be to perform the excavation or dumping operation.
Referring to FIG. 1, generally, an excavator 100 is constructed such that a work apparatus including a bucket 2, an arm 3, and a boom 4 is pivotally mounted on an upper swing structure 1. In particular, the upper swing structure 1 is rotated in a clockwise or counterclockwise direction about a turning-joint (not shown) in response to an operator's swing operation.
The bucket 2 receives a load due to the weight of soil contained therein, and the upper swing structure 1 receives a large moment of inertia while being turned during the excavation or dumping operation.
The reason for this is that the bucket 2 is turned in a state of being away from the center of gravity of the upper swing structure 1 in a structural aspect. Further, as a load applied to the bucket 2 and a distance between the bucket 2 and the center of gravity of the upper swing structure 1 are larger, the moment of inertia is also larger.
In a state in which the bucket 2 is filled with soil, the moment of inertia generated upon a swing movement of the upper swing structure 1 is relatively large as compared to that in a state in which the bucket 2 is empty.
The moment of inertia may vary depending on the load applied to the bucket 2 as well as the inclined angle of the boom 4 with respect to a horizontal line. In addition, the larger the moment of inertia is, the larger vibration or the period of vibration is as well known in the art
Thus, in the case where the swing operation of the upper swing structure 1 is repeatedly performed in the clockwise or counterclockwise direction, the upper swing structure 1 receives an abrupt shaking or jerking shock and an operator who controls the work apparatus within a cab 5 also feels a sudden shock and shaking at a time period when the swing movement is started or stopped.
Namely, when the construction machine moves to swing the upper swing structure during the swing start or stop operations, the swing motor and the control valves associated with the swing movement is opening and closing too fast and hydraulic fluid is starting and stopping so fast. It causes the occurrence of a big shaking or jerking movement due to the moment of inertia of bucket load and the upper swing structure.
Under such a situation, the excavation or dumping operation continues to be performed, manipulability of the work apparatus will be considerably deteriorated and work efficiency will also be adversely affected.
Various attempts have been made to solve the aforementioned disadvantages. For example, Korean Patent Laid-Open Publication No. 2001-0057430 discloses a swing control device for hydraulic construction equipment in which the moment of inertia corresponding to swing acceleration or swing deceleration of the work apparatus varies depending on a change in the angle of a boom so as to solve a problem associated with an increase of a shock applied to the equipment when the swing operation is started or stopped.
In the above patent, there is disclosed a technology in which as the angle of the boom is changed, a controller generates a current signal corresponding to a change in the angle of the boom for application to a proportional pressure control valve so as to make the pilot pressure for the swing control variable and make variable the operation of a direction control valve adjusting the flow path to supply a fluid to a swing motor according to a variation of the pilot pressure, so that a degree of acceleration or deceleration is controlled upon the start or stop of the swing operation to reduce the occurrence of the abrupt movement upon the quick stop or quick swing.
However, such a conventional technology entails a disadvantage in that since the movement of inertia of the upper swing structure varying depending on the angle of the boom and the variable control of the direction control valve for the swing operation must be taken into consideration, a hydraulic circuit is complicated in which a boom angle sensor and a pressure reducing valve are combined.